Injection molding is a well-known process for molding plastic parts. One application of injection molding is the formation of electrical connectors. The newly emerging field of electrical connector products requires very tightly toleranced elastomers. As illustrated in FIG. 1, molded elastomers 10 used in electrical connectors are often vulcanized to a carrier frame 12 to enhance the dimensional stability of the elastomer. The elastomer mold apparatus 100 has an upper mold 20, a lower mold 22, and a mold cavity 24 defined between upper mold 20 and lower mold 22 and adapted to receive molded elastomer 10 and carrier frame 12. Upper mold 20 is typically designed to locate on a datum surface 16 of carrier frame 12 to mold critical design elastomer features to the smallest tolerance possible. Datum surface 16 is located on the top of steps 14, which are integrally formed as part of carrier frame 12, along datum line A--A.
Upper mold 20 has partitions 26 which form the space 28 which accepts the material of elastomer 10. The critical height dimension of elastomer 10 is controlled with reference to datum surface 16. Upper mold 20 seats on datum surface 16, without compressing datum surface 16, to define the height of elastomer 10. Partitions 26 must not contact carrier frame 12 when upper mold 20 contacts datum surface 16; otherwise, undesirable deformation of carrier frame 12 might occur. Upper mold 20 cannot simultaneously contact two surfaces while still retaining sufficient control over the height of elastomer 10. Consequently, a clearance 30 exists between partitions 26 and carrier frame 12.
Flashing of the elastomer material may occur, at non-datum surfaces of carrier frame 12 such as the surfaces under partitions 26 of upper mold 20, because partitions 26 do not "shut off" or locate on these non-datum surfaces to seal the flow of elastomer during the injection molding process. Clearance 30 created by the failure to seal the flow of elastomer allows protrusions 40, known as "flash," on the finished article which must then be removed in a separate operation. Flash 40 is difficult to remove from the finished molded part. Typically, flash 40 is removed by a labor-intensive, manual process which adds cost to the part and poses a risk of handling damage.
The problem of flash has been the subject of a number of corrective attempts. Known attempts have been directed toward changes in the design of one or more mold components. Four such attempts are summarized below.
U.S. Pat. No. 5,597,523 issued to Sakai et al. discloses a molding apparatus (10) and method in which a mold cavity gasket is deformed by separately applied pressure to prevent flash formation. The molding apparatus has upper and lower molds (12 and 14, respectively), both made of metal, and a mold cavity (16) defined between the molds and adapted to receive a molding material such as epoxy resin. Due to manufacturing tolerances, a clearance on the order of a few microns tends to remain between the upper and lower molds in the clamped position--particularly when the molds are unheated. If such a clearance exists, the molding material may flow out of the mold cavity and into the clearance. When this occurs, undesirable flash will result.
To avoid flash, an annular recess (20) is defined adjacent to the mold cavity. A gasket (22) is fit in the recess and made of a deformable material such as lead. The gasket may also be made of synthetic resin, plastic, or other electrically insulative organic materials. A passage (24) is defined in the lower mold with one end of the passage communicating with the recess and the other end connected to a pump (26). A pressure medium such as silicon oil (25) fills the passage and exerts pressure on the gasket. This pressure deforms the gasket sufficiently to form a seal around the mold cavity. Although the seal prevents flash, the molding apparatus must be modified to create a recess and a passage in the mold, to accommodate a gasket, and to incorporate a pump and oil.
U.S. Pat. No. 5,543,159 issued to Iltgen discloses a flash-proof reaction-injection-molding (RIM) mold and a method of making the mold. RIM molds, like many molds, have at least two mold segments (14, 16) which, in a closed position, come together to define a mold cavity (8) and into which the reactants (2, 4) are injected. In a mold-open position, the mold permits removal or ejection of the molded article from the mold cavity. The mold segments each have a surface (20, 22) which faces the other mold segment. The surfaces have complementary shapes and come together, in the mold-closed position, along a plane known as a parting line (12) (thus, the parting line is analogous to the datum surface 16 of FIG. 1).
There is usually some degree of mismatch between the mold segment surfaces. This mismatch results in the formation of small gaps between the mating surfaces at the parting line, and particularly at the edge (24) of the parting line which is exposed to the mold cavity. When such gaps occur at the edge of the parting line, the liquid mixture injected into the mold cavity can invade the parting line at its edge and produce protrusions (i.e., flash) on the finished article which must then be removed in a separate operation.
To address the flash problem, Iltgen provides an interlayer film (18') of thermosetting resin between the mating mold surfaces. The resin fills any small gaps between those surfaces to prevent intrusion of the parting line by any liquid reactants injected into the mold cavity. The thermosetting resin adheres firmly to the surface (20) of one of the mold segments. Preferably, the surface of the mold segment to which the resin film adheres is roughened to provide a multiplicity of anchoring sites for the film. Again, although Iltgen addresses the problem of flash, his solution requires modification of the RIM mold to include a roughened mold surface to which a thermosetting resin adheres.
U.S. Pat. No. 4,686,073 issued to Koller discloses a mold modification similar to that of Iltgen. In his device for casting electric components on a terminal carrier plate (4), Koller incorporates a silicon rubber layer (2) which is cast on the surface of a flat, lower mold component (1). The silicon rubber layer is about 1 mm thick and contains recesses (3) which closely correspond to the dimensions of and accommodate the terminal carrier plate. A central mold component (5) is clamped down onto the lower mold component. The central mold component contains chambers (7). A ridge (8) surrounds the port which defines the bottom of each chamber. These chambers match the electrical components which are to be cast. An upper mold component (9) has openings (10) through which the casting resin (11) passes.
When the central mold component is clamped down, the silicon rubber layer is deformed by the ridges to seal the side surfaces (12) of the terminal carrier plate. Consequently, casting resin cannot penetrate the underside (13) and the side surfaces of the terminal carrier plate during casting. Although Koller does not address the problem of flash, he does propose a mold designed to prevent undesirable flow of casting resin. The mold design has a ridge and a cast-on silicon rubber layer.
U.S. Pat. No. 5,118,271 issued Baird et al. is directed to an apparatus for encapsulating a semiconductor device. The mold of the apparatus has a first cavity plate (17) and a second cavity plate (13). A semiconductor lead frame (10) to be encapsulated is placed between the cavity plates. The mold is closed so that the clamping surfaces (23) of the cavity plates clamp directly onto the lead frame. Once the mold is closed, an encapsulating material is introduced into the mold to encapsulate the lead frame. Elastic seals (19, 21) surround the outer surface of each cavity plate but do not cover either the clamping surfaces or the inner surfaces of the cavity plates. After the mold is closed, the elastic seals are pressurized and deform to compensate for any dimensional variations of the mold cavity plates or lead frame and completely seal the space between leads. Therefore, the elastic seals provide a supplementary seal to the clamping surfaces of the cavity plates and a primary seal in the space between leads of the encapsulated lead frame. These seals prevent encapsulating material from escaping.
Like the Koller device, the apparatus of Baird et al. does not address the problem of flash. Baird et al. propose a mold designed to prevent undesirable flow of encapsulating material. The mold design has elastic seals surrounding the outside surface of the mating mold cavity plates.
To overcome the shortcomings of prior attempts to address the problem of flash formation during the process of molding an elastomer to a substrate such as an electrical connector, a new, deformable, elastomer molding seal for use in an electrical connector system is provided. An object of the present invention is to provide a molding seal. Another object is to mold a plastic or metal substrate without producing undesirable flash, thereby eliminating the need for a costly process step which removes elastomer flash. A related object is to eliminate a flash removal step while avoiding modifications to the mold. Another related object is to reduce the cost of manufacturing a molded part.
It is still another object of the present invention to improve the quality of the completed, molded part by eliminating the risk of handling damage which arises when the part is exposed to the process step of flash removal. Yet another object of this invention is to extend the life of the mold used to manufacture the molded part. An additional object is to achieve these advantages while still allowing the mold to accurately register the height of the molded elastomer to any surface of the substrate desired by proper mold design. Finally, an object of the present invention is to meet the need for very tightly toleranced elastomers demanded by the newly emerging field of electrical connector products.